OH in Rutile: An Oxygen and Water Barometer

金红石中的 OH：氧气和水晴雨表

• 批准号: 0538107
• 负责人: Elizabeth Johnson
• 金额: $ 24.91万
• 依托单位: University of California-Los Angeles
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2006
• 资助国家: 美国
• 起止时间: 2006-05-01 至 2009-04-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0538107&HistoricalAwards=false
• 关键词: OH; Rutile; Oxygen; Water; Barometer

Dehydration of the subducting lithosphere induces oxidation and partial melting in the mantle wedge above subduction zones, and storage of water in the form of hydroxyl in high-pressure mineral phases may be an important mechanism for transfer of water to the mantle. Recently there has been much interest in the possible existence of a hydrosphere on the very early (4.3 Ga) Earth, highlighting the need to learn more about the fluid composition and oxygen fugacity of rocks that can provide important information about the early terrestrial oxidation state and water cycle. Oxygen fugacity and water activity are difficult to measure or infer unless one can find coexisting oxygen- or water-buffering mineral assemblages. This project will experimentally investigate the relationship between OH concentrations in rutile and the imposed oxygen fugacity, water activity, ferric iron content, temperature, and pressure, and will use a thermodynamic model to apply these results to geologic systems for which constraining oxygen fugacity and fluid composition is important, but has been difficult to evaluate with other methods. The technique is based on OH concentration measurements in rutile (TiO2), a commonly occurring accessory mineral that is nominally anhydrous but can incorporate a significant amount of hydroxyl into its structure. In pure rutile, H+ is stoichiometrically incorporated into the structure via charge-coupled reduction of Ti4+ to Ti3+ (resulting in a change in color from pale yellow to deep blue). Diffusion experiments of hydrogen in rutile will result in quantification of equilibrium boundary conditions and rates of redox-driven and intrinsic defect-driven loss of hydrogen in nominally anhydrous minerals. A new diffusion model will be constructed from this data to resolve the seemingly contradictory results from previous hydrogen diffusion experiments in nominally anhydrous minerals and measurements of OH in natural samples. Once developed, the rutile oxygen and water barometer should have widespread applications in geochemistry and petrology. Water activity or oxygen fugacity will be able to be determined even in systems lacking hydrous minerals or traditional oxygen buffering mineral assemblages. The proposed research will allow the PI, an early-career female scientist, to carry on active research in an academic setting. This grant will support undergraduate research aids over two summers.

俯冲岩石圈脱水导致俯冲带上方地幔楔的氧化和部分熔融，高压矿物相以羟基形式储存水可能是水向地幔转移的重要机制。 最近有很多兴趣在可能存在的水圈在非常早期（4.3 Ga）的地球，突出需要了解更多的流体成分和氧逸度的岩石，可以提供有关早期陆地氧化状态和水循环的重要信息。 氧逸度和水活性很难测量或推断，除非人们可以找到共存的氧或水缓冲矿物组合。 该项目将通过实验研究金红石中OH浓度与氧逸度、水活度、三价铁含量、温度和压力之间的关系，并将使用热力学模型将这些结果应用于地质系统，其中限制氧逸度和流体成分很重要，但很难用其他方法进行评估。 该技术基于金红石（TiO 2）中的OH浓度测量，金红石是一种常见的辅助矿物，名义上是无水的，但可以将大量的羟基结合到其结构中。 在纯金红石中，H+通过Ti4+到Ti3+的电荷耦合还原以化学计量方式并入结构中（导致颜色从浅黄色到深蓝色的变化）。氢在金红石中的扩散实验将导致在名义上无水矿物中的氢的氧化还原驱动和内在缺陷驱动的损失的平衡边界条件和速率的量化。 一个新的扩散模型将从这些数据来解决看似矛盾的结果，从以前的氢扩散实验在名义上无水矿物和测量的OH在天然样品。 金红石型氧水气压计一旦研制成功，将在地球化学和岩石学中有广泛的应用。 即使在缺乏含水矿物或传统的氧缓冲矿物组合的系统中，也能够确定水活度或氧逸度。 拟议的研究将使PI，一个早期的职业女性科学家，在学术环境中进行积极的研究。 这笔赠款将支持本科研究援助超过两个夏天。